1. Field of the Invention
The present invention relates to a power steering system for generating an auxiliary steering force for the steering of a vehicle such as a four-wheel-drive car or the like.
2. Description of the Related Art
FIG. 16 shows an example of a power steering system. The power steering system as shown in FIG. 16 is disclosed in Japanese Patent Application, First Publication, No. Hei 9-84300, in which output shaft 2 of driving unit (i.e., motor) 1 for generating the above auxiliary steering force and input shaft 3 coupled to the steering unit of a vehicle are coupled via torque limiter 4. This torque limiter 4 comprises cylindrical limiter cover (or case) 5 having a bottom, fixed in a manner such that the cover is rotationally driven together with the output shaft 2, similar cylindrical cover 6 having a bottom, which covers from the opening side to the outer surface of the limiter cover 5, limiter plate (or boss) 7 which is arranged to be relatively rotatable with respect to limiter cover 5 and is coupled to the input shaft 3, and friction plate 9 which is arranged between cover 6 and limiter plate 7 via forcing member (or spring member) 8 so as to be driven together with the limiter cover 5 and which is relatively rotatable with respect to the limiter plate 7.
The above cover 6 is attached on limiter cover 5 after limiter plate 7, forcing member 8, and friction plate 9 are set in the limiter cover 5. The cover 6 is fixed by folding down and clamping the peripheral edge of its opening side, and forcing member 8 comes in contact with the bottom of the cover 6, by which pressing force by this forcing member 8 is supported. In the power steering system having the above-described structure, during ordinary steering, auxiliary steering force generated from driving unit 1 is properly transmitted from output shaft 2 via torque limiter 4 to input shaft 3 by frictional force generated between limiter cover 5, friction plate 9, and limiter plate 7 caused by pressing force of forcing member 8. In a case in which impact force is transmitted from the wheel side of the steering unit, a slip is generated between limiter cover 5, friction plate 9, and limiter plate 7 of torque limiter 4, and impact torque is absorbed; thus, it is possible to prevent excessive torque from acting on output shaft 2 of driving unit 1.
In torque limiter 4 in such a power steering system, a predetermined frictional force is generated between limiter cover 5, friction plate 9, and limiter plate 7, as described above; thus, it is necessary to ensure sufficient contact areas between these members. Accordingly, it is inevitable that the outer diameter of torque limiter 4 is considerably large. However, in a torque limiter having such a large outer diameter, which is rotated together with output shaft 2 and input shaft 3, the moment of inertia of this torque limiter 4 also becomes large, and there occur problems in that the driving force necessary for rotationally driving output and input shafts 2 and 3 is increased, and follow-up capability between rotation of input shaft 3 and rotation of output shaft 2 is degraded. In particular, in the above conventional power steering system, cover 6 is arranged to cover the outer surface of limiter cover 5. Therefore, the outer diameter of torque limiter 4 is further increased and the moment of inertia is also increased; thus, the above-mentioned problems are assumed to be very substantial.
Also in torque limiter 4 in the above power steering system, it is required that during ordinary steering, auxiliary steering force be properly transmitted from output shaft 2 to input shaft 3, while when impact force acts thereon, impact torque is securely absorbed, as described above. Therefore, pressing force caused by the forcing member 8 must be strictly controlled so as to set the friction force between limiter cover 5, friction plate 9, and limiter plate 7 to be in a predetermined range. However, in the above conventional power steering system, the pressing force caused by the forcing member 8 is supported by cover 6 which covers limiter cover 5, and the opening of this cover 6 is simply clamped and fixed to limiter cover 5. Therefore, the support position of forcing member 8 is liable to change according to clamping conditions, and there is also a possibility that cover 6 will shift due to spring-back and deformation of the clamped opening of cover 6, by which support of forcing member 8 becomes unstable. According to the above possibilities, there occurs another problem in that predetermined frictional force is not provided between limiter cover 5, friction plate 9, and limiter plate 7.
In consideration of the above circumstances, the present invention provides a power steering system based on a first objective to reduce the moment of inertia of the torque limiter, and on a second objective to accurately determine pressing force by a forcing member.
In addition, the present invention has a further objective to improve efficiency of forming a limiter plate as a constituent of the torque limiter of the power steering system.
In order to realize the above objectives, the present invention provides a power steering system in which an output shaft of a driving unit and an input shaft of a steering unit are coupled via a torque limiter, the torque limiter comprising: a substantially-cylindrical rotating member which is rotatable together with one of the output shaft and the input shaft; and a rotated member which is rotatable together with the other of the output shaft and the input shaft, and which is forced toward the rotating member side by a forcing member supported by the rotating member.
Regarding the above basic structure, a typical example is such that the rotating member is a substantially-cylindrical limiter cover with a bottom, and the rotated member is a limiter plate contained in the limiter cover.
In the above typical example, it is possible for the forcing member to be supported by a concave portion formed in an inner-peripheral area of the limiter cover.
According to such a structure, the forcing member is supported by a concave portion formed in an inner-peripheral area of the limiter cover; thus, in contrast to the above-mentioned conventional power steering system, it is unnecessary to provide a cover (6) for supporting the forcing member (8). In this case, it is possible to prevent the outer diameter of the torque limiter from exceeding the outer diameter of the limiter cover and to regard the outer diameter as that of the torque limiter. Therefore, the moment of inertia of the torque limiter can be reduced. Accordingly, the driving force of the driving unit can be reduced and rapid follow-up between rotation of the input shaft and rotation of the output shaft can be realized, and good steering response can be obtained.
In the conventional structure which includes a cover (6) attached to the limiter cover (5) by clamping the opening portion of the cover (6), the position where the forcing member is supported may be shifted depending on clamping conditions, or due to deformation by spring-back, by which support of the forcing member becomes unstable. In contrast, according to the support by providing the concave portion formed in the limiter cover as described above, the above problems can be prevented, and it is possible to accurately position the forcing member in the limiter cover and to easily and accurately set the relevant force to be a predetermined size. Consequently, during ordinary steering, the output shaft and the input shaft can be stably rotated integrally, while if an impact force acts on the steering unit side, it is possible to more reliably generate sliding and to prevent excessive torque from being transmitted.
In the concave portion, a ring-shaped member having an inner-diameter smaller than the diameter of the inner-peripheral area may be fit, and the forcing member is supported via the ring-shaped member. In this arrangement, the limiter plate can be more stably forced in comparison with an arrangement in which a portion fit to the concave portion is provided in the forcing member so as to directly support the forcing member via the concave portion. In this case, it is desirable that the concave portion be circularly formed in the inner-peripheral area of the limiter cover, and the ring-shaped member be a C-ring having a slightly reduced diameter and which is forced toward its expanding directions when placed in the concave portion. In this structure, the mounting strength of the ring-shaped member can be improved according to the force generated by expansion of the C-ring; therefore, it is possible to more accurately position and stably support the forcing member.
On the other hand, in the above typical example of the present invention, the forcing member may be supported by a projecting portion which is formed by deforming the cylindrical portion of the limiter cover toward the inside.
According to this structure, the forcing member is supported by a projecting portion formed by deforming the cylindrical portion of the limiter cover toward the inside; thus, also in this case, it is unnecessary to provide an additional cover for supporting the forcing member. Therefore, the outer diameter of the torque limiter can be prevented from exceeding that of the limiter cover. In addition, as the above projecting portion is formed by deforming the cylindrical portion of the limiter cover from the outside toward the inside, the moment of inertia of the limiter cover can be reduced by the weight of this projecting portion. According to these features, the moment of inertia of the torque limiter can be generally reduced. Consequently, the driving force of the driving unit can be reduced and rapid follow-up between rotation of the input shaft and rotation of the output shaft can be realized, and good steering response can also be obtained in this case.
It is also possible that:
(i) the projecting portion be formed by shearing a part of the cylindrical portion in a radial direction along a circumferential line and simultaneously pressing the sheared part toward the inside, or that
(ii) the projecting portion be formed by deforming a flexible portion, which is formed by a cut provided in a circumferential line of the cylindrical portion of the limiter cover, toward the inside in a radial direction of the cylindrical portion.
In these cases, the projecting portion can be provided by deforming the sheared part of the cylindrical portion, or the flexible portion formed using the cut, not in (parallel with) the center-axis direction of the limiter cover, but only toward the inside. Therefore, in comparison with the conventional case in which the opening portion of a cover attached to the limiter cover is fold up and clamped, shifts of the position, where the forcing member is supported, depending on the clamping conditions, can be prevented. In this case, even if spring-back occurs, the projecting portion is deformed only toward the outside in a radial direction of the limiter cover. Therefore, it is possible to stably support the forcing member and accurately position the forcing member in the limiter cover, and pressing force by the forcing member can be easily and accurately set to be a predetermined size. Consequently, also in this arrangement, during ordinary steering, the output shaft and the input shaft can be stably rotated integrally, while if an impact force acts on the steering unit side, it is possible to more reliably generate sliding and to prevent excessive torque from being transmitted.
Typically, in the above arrangements in which the projecting portion is formed by shearing and deforming the cylindrical portion, or by deforming a flexible portion formed using a cut, a plurality of the projecting portion are separately positioned in a circumferential line of the limiter cover. In this case, it is preferable that a disk-shaped plate be placed between the projecting portions and the forcing member, by which the forcing member can be more stably supported via the whole circumferential line in the plate even though the projecting portions are separately positioned.
In each arrangement described above, by coaxially coupling one of the output shaft and the input shaft and the limiter plate via a metal bush so as to enable relative rotation, the other of the output shaft and the input shaft which is rotatable together with the limiter plate can be coaxially arranged with the one of the output shaft and the input shaft, that is, xe2x80x9ccoaxiabilityxe2x80x9d between the output shaft and the input shaft can be established.
Furthermore, in the above basic structure, the following arrangement is also possible, that is, in the outer-peripheral surface of the other of the output shaft and the input shaft, plural splines are formed in parallel with an axial direction of the relevant shaft; the rotated member separately comprises:
(i) a cylindrical portion, on the inner-peripheral surface of which plural splines are formed in parallel with its axial direction, these splines being engaged with the splines formed in the outer-peripheral surface of the other of the output shaft and the input shaft; and
(ii) a flange portion, which is forced toward the rotating member side, by which the flange portion functions as a friction clutch.
Regarding the above engaged sets of splines, one functions as the keyways while the other functions as the keys.
In the torque limiter in the power steering system as structured above, (i) the cylindrical portion, having the splines on the inner-peripheral surface, to which a shaft having splines which can be engaged with above splines is inserted, and (ii) the flange portion which must have a suitable accuracy for functioning as a friction clutch, are separately formed. Therefore, efficient manufacturing is possible in consideration of accuracy requirements relating to formation of each portion. In addition, both portions may be made of different materials; thus, efficiency of forming the limiter plate can be improved and relevant accuracy and strength conditions can be respectively improved.
The cylindrical portion and flange portion may be coupled via a bush, or may be fixed to each other by fitting the side of one end of the cylindrical portion to the flange portion. In the latter case, for example, a concave portion is formed at the flange portion side of the cylindrical portion, to which (concave portion) the flange portion (that is, a plate portion) is fit, and the side of the one end of the cylindrical portion is clamped and fixed to the flange portion.
If the side of the one end of the cylindrical portion is directly fit to the flange portion, a cap is desirably inserted into the cylindrical portion because the area from the side of the one end of the cylindrical portion up to the flange portion is vacant.